No property sale record on MLS?

[Alex G.]

I am doing the due diligence on a duplex in Central Maine (https://www.realtor.com/realestateandhomes-detail/46-E-School-St_Oakland_ME_04963_M42271-18089?view=qv) trying to determine how much the seller paid for the property in 2006. The town assessor puts the number at $80,000 citing public record. The Kennebec County Registry of Deeds has a contemporaneous mortgage record. However, my agent says that MLS has no record of the sale.

Does this mean the property was not MLS-listed and changed hands by way of off-market sale?